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When is a CDATA section necessary within a script tag?

Are CDATA tags ever necessary in script tags and if so when?

In other words, when and where is this:

<script type="text/javascript">
//<![CDATA[
...code...
//]]>
</script>

preferable to this:

<script type="text/javascript">
...code...
</script>

Answer*

## HTML ##
An HTML parser will treat everything between `<script>` and `</script>` as part of the script. <strike>Some implementations don't even need a correct closing tag; they stop script interpretation at "`</`", which is correct according to the [specs][1]</strike>. 

> **Update** In HTML5, and with current browsers, that is not the case anymore.

So, in HTML, this is *not* possible:

 <script>
 var x = '</script>';
 alert(x)
 </script>

A `CDATA` section has **no effect at all**. That's why you need to write

 var x = '<' + '/script>'; // or
 var x = '<\/script>';

or similar.

This also applies to XHTML files served as `text/html`. (Since IE does not support XML content types, this is mostly true.)

## XML ##

In XML, different rules apply. Note that (non IE) browsers only use an XML parser if the XHMTL document is served with an XML content type.

To the XML parser, a `script` tag is no better than any other tag. Particularly, a script node may contain non-text child nodes, triggered by "`<`"; and a "`&`" sign denotes a character entity.

So, in XHTML, this is *not* possible:

 <script>
 if (a<b && c<d) {
 alert('Hooray');
 }
 </script>

To work around this, you can wrap the whole script in a `CDATA` section. This tells the parser: 'In this section, **don't treat "`<`" and "`&`" as control characters**.' To prevent the JavaScript engine from interpreting the "`<![CDATA[`" and "`]]>`" marks, you can wrap them in comments.

If your script does not contain any "`<`" or "`&`", you don't need a `CDATA` section anyway.


 [1]: http://www.w3.org/TR/html401/appendix/notes.html#notes-specifying-data

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3

The statement "A CDATA section has no effect at all" is not true for (the proposed) HTML5, which recognizes the construct. w3.org/TR/html5/syntax.html#cdata-sections

danorton
– danorton

2012年03月06日 20:29:30 +00:00
Commented Mar 6, 2012 at 20:29
3

@danorton Interesting. I think that's a pretty ugly mix. Still no effect in script content though.

user123444555621
– user123444555621

2012年03月06日 22:06:39 +00:00
Commented Mar 6, 2012 at 22:06
2

Did not know that any </ inside script tags is bad.

Salman Arshad
– Salman Arshad

2013年03月05日 18:38:54 +00:00
Commented Mar 5, 2013 at 18:38
3

@SalmanA That's one of HTML's odditys and officially called ETAGO. Learn more: mathiasbynens.be/notes/etago (while the article states that no browser ever implemented that feature, I'm pretty sure it caused some trouble for me. Maybe in some other tool)

user123444555621
– user123444555621

2013年03月05日 20:35:22 +00:00
Commented Mar 5, 2013 at 20:35
1

Actually I ran into validation problems -- <script>var b = "<b>bold</b>";</script> fails to validate but after reading your answer and changing to <script>var b = "<b>bold<\/b>";</script> fixed it.

Salman Arshad
– Salman Arshad

2013年03月05日 21:52:21 +00:00
Commented Mar 5, 2013 at 21:52

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When is a CDATA section necessary within a script tag?

Answer*